A conventional rotational speed sensing device is disclosed in Japanese Utility Model Laid-open publication No. 5(1993)-94771 published without examination on Dec. 24, 1993. The rotational speed sensing device disclosed in this document is illustrated in FIG. 9 and FIG. 10.
As shown in FIG. 9, the rotational speed sensing device includes a pulsar ring 104, a permanent magnet 105, a coil 108 and a magnetic member 106. The pulsar ring 104 is fixed to a rotating member 101 and the rotating member 101 is rotatably supported on a cylindrical member 103 via a plurality of balls 102. The cylindrical member 103 is fixed to a shaft 100. As shown in FIG. 10, the pulsar ring 104 has a plurality of generally equally spaced apart projections 104a, with the projections being disposed in the circumferential direction of the pulsar ring 104.
The permanent magnet 105 is disposed between the pulsar ring 104 and the cylindrical member 103 and is fixed to a housing 111 that is fixed to the cylindrical member 103. The permanent magnet 105 has an N-pole for generating magnetic flux and an S-pole for receiving magnetic flux generated by the N-pole.
The coil 108 is disposed between the pulsar ring 104 and the cylindrical member 103 and is formed around a bobbin 107 that is fixed to the housing 111. The coil 108 substantially opposes the permanent magnet 105 and generates a voltage corresponding to the magnetic flux which passes through the inner circumferential side of the coil 108 in the axial direction of the rotating member 101. One end of the coil 108 is connected to a first terminal 109 and the other end of the coil 108 is connected to a second terminal 110.
The magnetic member 106 is disposed at the inner circumferential side of the coil 108 and is fixed to the housing 111. The magnetic member 106 is provided for forming a magnetic circuit with the permanent magnet 105, the pulsar ring 104 and the inner circumferential side of the coil 108. As shown in FIG. 10, the magnetic member 106 has a plurality of projections 106a corresponding to the projections 104a on the pulsar ring 104. The magnetic member 106 is alternately located relative to the pulsar ring 104 in a first position in which the distance between each projection 106a and each wall portion 104a is relatively small and in a second position in which such distance is relatively large.
The operation of the convention device described above is as follows. When the rotating member 101 is rotated, the pulsar ring 104 rotates with the rotating member 101. Since the magnetic member 106 is alternately positioned relative to the pulsar ring 104 in the first and second positions during this time, the magnetic resistance between each projection 106a of the magnetic member 106 and each projection 104a of the pulsar ring 104 varies, whereby magnetic flux which passes through the inner circumferential side of the coil 108 in the axial direction of the rotating member 101 varies. As a result, the coil 108 generates voltage corresponding to the magnetic flux which passes through the inner circumferential side of the coil 108 and this voltage is picked up by the terminals 109, 110 so that the rotational speed of the rotating member 101 can be detected.
In the above-described device, since the magnetic circuit which includes the inner circumferential side of the coil 108 is only formed by the magnetic member 106 when the magnetic member 106 is located relative to the pulsar ring 104 in the second position, magnetic flux which passes through the inner circumferential side of the coil 108 may be fully decreased during this time. In other words, the difference between the magnetic flux which passes through the inner circumferential side of the coil 108 when the magnetic member 106 is located in the first position relative to the pulsar ring 104 and the magnetic flux which passes through the inner side of the coil 108 when the magnetic member 106 is located relative to the pulsar ring 104 in the second position is relatively small. Thus, the output of the coil 108 is relatively small.
To increase the output of the coil 108, the permanent magnet 105 must be enlarged or the number of turns of the coil 108 must be increased. Unfortunately, this makes the rotational speed sensing device undesirably larger and more expensive.